1. Technical Field
The present invention relates generally to a coating system and in particular relates to a method and apparatus for curtain coating a surface. More specifically, the present invention relates to a coating system wherein the physical condition or state of a continuously flowing curtain of coating composition is determined using sound or light pulses or waves.
2. Background Art
Ultrasonic ranging and detection devices utilize high frequency sound waves to measure distances and detect objects. This is done by reflecting high frequency sound waves from objects and by the interruption of high frequency sound waves by an object to be detected. Depending upon the type of sensor that is used, at least one or two transducers are required.
When two transducers are used, one transducer is used as a transmitter to transmit a continuous series of ultrasonic pulses and the second transducer is used as a receiver to receive the transmitted ultrasonic energy. For example, two transducers can be mounted side-by-side and when ultrasonic energy from the transmitter is reflected back from an object, the receiver receives the ultrasonic energy. Some transducers both transmit and receive sonic energy through the same transducer, and with such transducers, only one transducer is required.
Whether one or two or more transducers are used, each transducer is connected to an electronic control module that receives electrical signals from an electronic monitoring module in each transducer and performs predetermined control functions. The control functions are based upon the electrical signals from the electronic monitoring module and include time lapse measurement and distance measurement, as well as other measurement functions.
The speed of ultrasonic energy through a given atmosphere at a relatively constant temperature is a constant. The speed of light, of course, is a constant. Thus, the period of lapsed time between the time an ultrasonic pulse is transmitted and the time a significant reflected ultrasonic pulse or echo is received is a direct function of the distance between the transducer or transducers and the object. This period of lapsed time is used to control an electronic switch or latch. For example, the electronic switch or latch can be set to an "off" position the moment an ultrasonic wave is transmitted, and when a significant echo of the ultrasonic wave is received, the electronic switch can be set to an "on" position. The length of time the electronic switch is in the "off" position can be converted into proportional (or inverted proportional) voltage and current as well as solid state DC on/off switching, depending on the length of time between the transmitting and the receiving of ultrasonic waves.
A transducer may be placed as close as two (2) inches from the object or as far away as sixty (60) inches and further. The level of noise filtration is adjustable using potentiometers, and thus the degree of sensitivity of the receiving transducer can be adjusted in order to effectively predetermine what level of received strength constitutes a significant echo.
Curtain coating methods and apparatus are well-known in the coating field and are widely used for manufacturing photographic films and other thin film coated products. U.S. Pat. No. 3,508,947 to Hughes is directed to a curtain coating method and apparatus for making photographic elements by applying a coating composition onto flexible supports or webs as a free-falling vertical curtain, and discloses the general concept of coating using a curtain of coating composition.
In the curtain coating process, the quality of coating is largely determined by the properties of the liquid curtain. It is critical to insure that a stable laminar flow of liquid film is formed by the slide hopper and that an equally stable, laminar flow liquid curtain is formed from that film To prevent contraction of the falling curtain under the effect of surface tension, it is known that the curtain width must be guided at its edges by curtain edge guides.
Curtain edge guides are generally solid, stationary members that are attached to the slide hopper and extend downwardly from the initial point of free fall of the curtain. Wetting contact of the edges of the falling curtain with the edge guides should be maintained the entire length of the edge guide to avoid a break in the curtain. An edge band liquid can be supplied to the lateral sides of the curtain to stabilize the curtain, and this liquid is collected and recirculated for reuse. U.S. Pat. No. 4,019,906 issued Apr. 26, 1977, discloses a curtain coating process using such edge band liquid techniques.
The edge portions of the film support are customarily embossed by a knurling wheel to increase the effective thickness of the support. The increased thickness of the edge protects the sensitive photographic materials from higher localized winding pressure which occurs as a result of minute variations in support thicknesses. It is desirable to avoid applying any coating compositions to the knurled areas because inherent tackiness of photographic coatings at high humidity makes it difficult to unwind the support during finishing operations. One way of achieving this is to maintain a free falling curtain having a width less than the width of the support. Thus, techniques for providing coating-free margins would be highly useful in connection with edge knurling of the support.
One prior art attempt to maintain the knurled edge portions coating-free has been to locate the edge guides in-board of the edge of the support. However, this causes the coating to terminate along both edges of the support in a bead. Such edges must be cut off to provide a completely uniform coated support. Prior to costly and wasteful trimming operations, however, such excessively thick beads of coating liquid can adversely affect drier efficiency and result in contamination of downstream transport rollers.
Another drawback associated with the curtain coating method is that the air flow adjacent the curtain can cause defective coating layers to be applied onto the supports or webs. It is virtually impossible to render a coating room free of air currents because of the air currents generated by the moving supports or webs. Air currents are further generated by the necessity of having to provide proper ventilation, the necessity of having operating and maintenance personnel enter and exit the coating room, as well as temperature differentials that can cause air currents in the coating room. Such air currents can cause defects in the coating layer, such as non-uniform layers, wrinkles and voids or breaks.
Various slightly redundant attempts have been made to alleviate this problem. For example, in U.S. Pat. No. 4,128,667 to Timson, a stream of gas directed over an air foil situated immediately adjacent a stream of free falling coating fluid is utilized to direct and alter the velocity of the fluid prior to its reaching the web being coated. The air foils guide the stream of gas along the entire width of the fluid stream and act uniformly over the entire length of the stream. In this manner, not only the velocity but the lateral movement of the fluid stream may be controlled. The air foils, however, do not protect the curtain and, in fact, would be damaging to a curtain of photographic emulsion although the damage to plain polymer coatings would be negligible where the character of the emulsion is not sensitive to such damage.
In U.S. Pat. No. 4,287,240 to O'Connor, a shield is provided adjacent the coating zone to protect the flow of coating compositions against disturbance by ambient air currents.
While these attempts have helped to somewhat alleviate the problems caused by disturbances in the ambient air, there still remains the problem that when a disturbance does cause irregular coating of the coating composition layer on the support or web, the defect may go unnoticed until the product reaches quality control check or, sometimes, until the product reaches the end-user.
Under current coating methods and operational parameters, where the web or support is traveling through the coating area at speeds of between one foot per second to several hundreds of feet per second and faster, a defect can be easily missed by simple visual inspection. In the case of continuous voids or gaps in the curtain, hundreds and even thousands of feet of unacceptable and unusable coated support or webbing may be produced before the defect is discovered and corrected. If the defect is discovered, the coating machine must be stopped and the take-up roll must be inspected and unrolled until the beginning point of the defect is reached, with the result that the defective coated support or webbing must be thrown away. This results in losses in terms of lost product and lost production time. If the defect is not discovered, the defective product becomes subject to further investment in terms of downstream processing and packaging, for example, into photographic films, cassette tapes, and other such products, and therefore causes further losses in terms of lost investment. Moreover, if the defective product somehow escapes quality control systems, an end-user may suffer losses as a result of reliance on the defective product.
Under current production schemes, the actual coating speed is limited primarily by the drier capacity. In a typical emulsion curtain coating operation, the coating speed is increased until the drier in the form of a long oven having air flow supports for the emulsion-coated substrates cannot effectively dry the emulsions. In such an operation, it is critical to maintain a proper and consistent coating thickness in order to avoid contaminating downstream rollers as a result of overcoverage of the substrate with emulsion. In such an operation, if the downstream rollers become contaminated with emulsion, hundreds, if not thousands, of feet of coated substrate could become damaged due to adhesion as well as random peeling of the emulsion coating from the substrate. Moreover, the downtime necessary to clean the rollers and other machine parts that have been contaminated with excess emulsion from overcovered substrate results in further substantial losses.
A curtain of coating composition most frequently breaks away from the bottom corner edges of the curtain guides due to the cohesive nature of the composition combined with the acceleration of the curtain as it falls. Occasionally, the curtain will break in the middle due to some compositional imperfection that temporarily breaks the curtain formation. In such a situation, there is almost certain overcoverage of at least certain sections of the substrate with the photographic emulsion. While such overcoverage may only be on a short length of substrate, if undetected, the excess emulsion will either spill off of the substrate or not dry properly and stick to the surface of downstream rollers and contaminate the rollers as well as other mechanical parts and cause the entire coating machine to be shut down for cleaning.
A need has existed for a method and apparatus for coating a surface of a continuously moving substrate with a continuously flowing curtain of coating composition wherein the physical state of the curtain is detected. A need has also existed for a method and apparatus for coating a surface of a continuously moving substrate with a continuously flowing curtain of coating composition wherein the physical presence of the curtain is detected. A further need has also existed for a method and apparatus for coating a surface of a continuously moving substrate with a continuously flowing curtain of coating composition wherein the position of the curtain is determined. Still a further need has existed for a method and apparatus for coating a surface of a continuously moving substrate with a continuously flowing curtain of coating composition wherein physical defects in the curtain are detected. Yet another need has existed for a method and apparatus for coating a surface of a continuously moving substrate with a continuously flowing curtain of coating composition that affords the early detection of physical defects in the curtain to allow effective, corrective action to be taken before such defects cause expensive damage. Another need that has existed is a method and apparatus for coating a surface of a continuously moving substrate with a continuously flowing curtain of coating composition that provides immediate detection of physical defects in the curtain to allow immediate corrective action.